A conventional ultrasonic imaging apparatus irradiates an ultrasonic wave generated from an ultrasonic vibration element in an ultrasonic probe to a patient, receives a reflected ultrasonic signal, by the ultrasonic probe, produced due to difference of sound impedance of tissue in the patient, and displays a ultrasonic image on a monitor. Since a 2-dimensional image is easily obtained in real time by putting the ultrasonic probe of the ultrasonic imaging apparatus on the patient, the ultrasonic imaging apparatus is widely used for morphologic diagnosis of various organs and functional diagnosis of a heart. In a conventional method of creating the ultrasonic image, an optimum ultrasonic frequency for a diagnostic part is selected, an ultrasonic wave pulse having a center frequency at the selected frequency is transmitted to the patient, and the reflective ultrasonic wave signal of almost equal to the center frequency is received to obtain the ultrasonic image.
In the meantime, a tissue harmonic imaging method (hereafter referred to as THI) has recently been developed, and the THI method spreads widely. The THI method uses a phenomenon of nonlinear ultrasonic propagation in a tissue. For example, when an ultrasonic wave of a center frequency (fo) is irradiated to a patient, a second harmonic component at twice the center frequency (2fo) is newly generated according to the phenomenon and is received by the ultrasonic probe with the basic ultrasonic wave at frequency (fo). A cause of generation of the harmonic component is that propagation speed of the ultrasonic wave pulse in the patient depends on sound pressure of the ultrasonic wave. For this reason, distortion of a received signal occurs and the harmonic component is generated. Generation of the harmonic component depends on status of the tissue, propagation distance to a reflective portion and ultrasonic intensity on the reflective portion. The dependence on the ultrasonic intensity can theoretically reduce generation of a side lobe which causes artifact on the ultrasonic image. Thus, using the THI method, a high resolution image can be obtained. It is difficult to appropriately extract the harmonic component from the received ultrasonic wave signal also including the basic component by a general filtering method, since parts of each of the frequency components overlap when each component has a wide frequency band.
As a method for extracting the harmonic component of wide frequency band, a pulse subtraction method has been developed, as described at page 53 of the 1989 publication by Ahirui et al., “Nonlinear Propagation of a Pulsed Ultrasound,” Shingaku-giho, University of Electro-Communications, US 89-23, pp. 53-60, 1989. In the pulse subtraction method, two ultrasonic wave pulses having different polarity are transmitted in a predetermined direction, and received signals are added to reduce the basic component and extract the harmonic component. The pulse subtraction method is based on the fact that the harmonic component is generated in proportion to the square value of the amplitude of the basic component. Therefore, when the transmitted ultrasonic pulse is reversed, the basic component is also reversed, but the harmonic component is not reversed. In the pulse subtraction method, in order to acquire the different polarity signals from the same portion in a time interval which it takes to transmit and receive a single ultrasonic wave (one rate section). For this reason, it is desirable that the reflective portion is still within the one rate section in order to appropriately reduce the basic component. Therefore, when the pulse subtraction method is applied to an actual patient, the basic component remains due to a motion of internal organs or a body, and so-called motion artifact occurs. To address such a problem, Japanese Patent Publication (Kokai) No. 2002-165796 (P. 4, FIGS. 2 through 5) discloses a method for reducing the remaining harmonic component that still remains even after the pulse subtraction by combining the pulse subtraction method and the filtering method. Japanese Patent Publication (Kokai) No. 2001-286472 (P. 5, FIGS. 6 through 9) discloses another method for reducing the remaining harmonic component by detecting the amount of movement of the body based on the received signals during a single rate section and by adding the signals corrected based on the amount of the movement.
Meanwhile, in ultrasonic diagnosis of a heart or an abdomen, a method for obtaining status of blood flow by detecting a reflective ultrasonic wave from an ultrasonic contrast media (hereafter referred to as contrast media) injected into a blood vessel, is known. As the contrast media, micro bubbles are generally used. Although a big reflective wave on the micro bubbles can be relatively obtained since an ultrasonic reflective coefficient of the micro bubbles is larger than that of the blood, the micro bubbles can be easily broken by irradiating an ultrasonic wave of usual energy. However, it is not desirable to use bubbles hard to be broken or to repeatedly use the micro bubbles, because such the use can impose much burden on the patient. Thus, it is difficult to observe for a while the status of the blood flow, since the contrast media using the micro bubbles disappears in a short time upon ultrasonic irradiation. To address such a problem, Japanese Patent Publication (Kokai) No. 8-336527 (pp. 3-5, FIGS. 1 through 3) discloses that the reflected wave on the contrast media is appropriately extracted by subtracting two received signals acquired when two kinds of ultrasonic waves are transmitted to and received from the same position of the patient. According to this method, a 1st ultrasonic transmission and reception is performed. At this time, a part of the ultrasonic pulse is reflected on the contrast media, and a 1st ultrasonic reflected wave is obtained. On the other hand, all or part of the contrast media where the ultrasonic pulse is reflected is broken. For this reason, when a 2nd ultrasonic transmission and reception is subsequently performed, the ultrasonic reflected wave on the contrast media (contrast media component) is smaller than that of the 1st transmission and reception, and the ultrasonic reflected wave on the tissue of the patient (tissue component) is the same size as that of the 1st transmission and reception. Therefore, when the subtraction is performed between the 1st ultrasonic reflective wave and the 2nd ultrasonic reflective wave, the contrast media component is extracted.
When an ultrasonic transceiver cycle (herein referred to as rate cycle) is shortened in order to reduce the motion artifact and to the improve real-time characteristic of the image by the increasing frame rate (number of images displayed per second), before the reflected ultrasonic wave that is reflected in a deep portion is received by the ultrasonic probe, the next ultrasonic pulse is irradiated. In this situation, if the pulse subtraction or the imaging of the contrast media is performed, the artifact remains since the ultrasonic reflected waves obtained by different ultrasonic transmission and reception are overlapped within the one rate cycle.
FIG. 1A and FIG. 1B show illustrations for explaining an artifact generated when the rate cycle Tr is shortened. A reflector 1 is the tissue of the patient, a reflector 2 is the contrast media, and a reflector 3 is the tissue of the patient. It takes longer to perform transmission and reception of the reflector 3 than the rate cycle Tr. In the case of a sector scan, the transmission and reception of a first scanning direction is performed in the rate section (1) and rate section (2), and the transmission and reception of a second scanning direction that is adjacent to the first scanning direction is performed in the rate section (3) and rate section (4) as shown in FIG. 1A. FIG. 1B shows a rate pulse for determining an irradiation timing of the ultrasonic wave, received signals from the reflector 1, 2 and 3, and results of subtraction between received signals obtained in the rate sections (1) and (2) and between received signals obtained in the rate section (3) and (4). The size of the received signal is indicated as size of arrow.
That is, the ultrasonic pulse irradiated in the scanning direction θ1 in rate section (1) is reflected on a reflecting point A1 of the reflector 1, a reflecting point A2 of the reflector 2, and a reflecting point A3 of the reflector 3. The reflected ultrasonic waves are detected by the ultrasonic probe 1 as the reflected intensities a11 through a31. Subsequently, the ultrasonic pulse irradiated in the same scanning direction θ1 in the rate section (2) is reflected on the reflecting points A1 through A3, and received signals of reflective intensities a12 through a32 are acquired. In this case, before the ultrasonic pulse of the rate section (1) reflects at the reflecting point A3 and is received by the ultrasonic probe 1, the ultrasonic pulse of rate section (2) is irradiated. For this reason, the reflected ultrasonic wave from the reflected point A3 of the rate section (1) is detected together with the reflective ultrasonic wave from the reflection points A1 and A2 of the rate section (2). Therefore, as shown in FIG. 1B, when the subtraction between the received signals of the rate section (1) and (2) is performed, the received signal from the reflector A1 is reduced, but the received signal from the reflector A3 remains. The reflective ultrasonic wave that remains like the above reflective ultrasonic wave from the reflector A3 is called a remaining echo.
On the other hand, the size a22 of the received signal from the contrast media obtained in the rate section (2) is remarkably reduced in comparison with the size a21 of the received signal of the rate section (1) since the contrast media is broken in the rate section (1). For this reason, amount of change a21−a22 is detected by the subtraction between the received signal from the contrast media in rate section (1) and the received signal from the contrast media in the rate section (2). When the subtraction, as well as the rate section (1) and (2), is performed between the received signals acquired in the rate section (3) and (4), the ultrasonic wave of the size a32−b31 based on the change of the remaining echo and the ultrasonic wave of the size b21−b22 are detected. Since the reflecting point B3 is different from the reflecting point A3, generally A32≠b31 and the remaining echo exists as well.